Iron alloy suitable for electrical resistances



Patented Aug. 6, 1940 ,PATENT OFFICE IRON ALLOY SUITABLE FOR ELECTRICAL RESISTANCES Thomas Swinden, Sheffield, England, assignor to Kemet Laboratories Compan'y,,Inc., a corporation of New York No Drawing.

g 1 Claim.

This invention relates to iron alloys which are especially suitable for the manufacture of electrical resistances.

It is known that various alloys of iron, alumi-.

6 num and carbon, with or without other metals such as chromium for example, are suitable for electrical resistance elements provided that the carbon content be very low, for instance below 0.05%. It is, however, a matter of expense and practical difliculty to maintain the'carbon content sufilclently low to insure that the resistance elements will withstand high. temperatures for a considerable period of time. It has been found that during annealing of the wire for further drawing, for instance, the carbon content may be raised from 0.05% to 0.1% or 0.15% by carburlzation, the carbon originating in the lubricant ordinarily used in wire drawing.

According to the present invention, it is found that the addition of titanium inan amountof. at least three times the carbon percentage, considerably improves the life of iron-chromiumaluminum alloys when used for the purpose of electrical resistance wire exposed to high temperatures, notwithstanding that the carbonco'ntent after drawing and annealing may lie between thelimits of about 0.1% and 1%. The titanium may be wholly or partially replaced if. desired by one or both of the elements vanadium or zirconium, the proportions of these elements being related to the carbon content.

The range of constituents contemplated is:

Chromium, 6% to 30% and preferably 6% to 20% Aluminum, 3% to 12% Carbon, 0.03% to 1% and preferably to 0.5%.

Suitable additions of one or more of the other elements mentioned may be made, which in the case of titanium and vanadium, should not be less than three times the percentage of carbon and in the case of zirconium not less than six times the percentage of carbon. The effects of these elements are approximately additive and whenmore than one is used, the preferred mini- 5 mum amounts will be determined according to the following formula:

. 3 I n It is preferred that the excess of titanium, vana dium or zirconium above'the minima. specified should not exceed 2%. Certain impurities may be present and we have found for instance that the presence of up to say 0.75% manganese and. upto say 0.30% silicon does not have any detri- Application June 5,v 1937, Serial In Great Britain June 8, 1936 mental effect. The total amount of the titanium, vanadium or zirconium or any of these metals taken singly need not exceed 5%.

In the following examples alloys of various percentage compositions manufactured in accordance with my invention are given, and for the sake of comparison alloys having a closely similar percentage composition but without the addition of titanium and also the results of actual tests carried out on S. W. G. wire in the bright drawn condition without subsequent heat treatment. These results serve to illustrate the vastly improved life in service obtained by means of the titanium additions. The resistivity is given in microhms perv cubic centimetre, and the coefficient of increase of resistance with temperature is given over a range of from 20 C. to 1,000 C. The life testis in hours when the wire under a standard load of lbs. per square inch was subjected to intermittent heating at a temperature of between 1,060 C. and 1,070 0., the heat being alternately two minutes on and two minutes off. In all the compositions the remaining constituent to make the full 100%consists substantially of. iron:

Example 1 Alloy according Closely similar to present alloy without invention, cast titanium, cast 8. G. 2007 S. G. 83

8. 44 8. 00 6. 42 6. 30 0. 085 0.13 0. 41 0. 32 0.22 0.18 0. Nil I 121 119 l. sax-10* 2. 6X10- Alloy ac- Closely similar alloy withcordlng to out titaniumthe present invention, cast 8. G. Cast 8. G. Cast 8. G. 1442 1441 1537 chromiumnflpercentl. 8. 04 8. 36 7. 92 Aluminum ..do 5. l4 4. 78 5. 50 d 0.10 0.10 0. 09 0. 5B 0. 48 0. 41 ,0. 23 0. 26 0. 14 0. 69 Nil Nil Resistivity 112 108 111 Temperature coeflicient 2.6X10- Life test l8 63 Too badly scaled during test to provide a reliable value.

Example 3 Alloy ac- Closely similar alloy withoording to out titaniumthe present invention, 7 Cast No. 59' Cast No. 61'

12. 84 13. 91 ll. 31 6. 30 6. 48 5. 98 0. 085 0. 06 0. 03 0. 62 i t 0. 30 t 0. 96 Nil Nil 126 139 132 Temperature coefl'lcient 1.4Xl0- f Lila test 853 158 189 Size 01' wire in mm; two instances=0.025". fNot determined.

It will be observed that the life of the alloy according to the present invention is very much lar compositions but not including titanium.

I declare that what I claim is: Electrical resistance element composed of 11 alloy wire containing chromium 6% to 30%; aluminum 3% to 12%; carbon ima. percentage sufl'icient detrimentally to affect the life of the element and between about 0.1% and 1%; and the balance principally iron and an efiective amount of. at least one element selected from the group titanium, vanadium, and zirconium, such amount being less than 5% but suflicient to overcome the detrimental effects of the carbon and being at least enough to satisfy the equation:

%3 +%6 carbon THOMAS SWINDEN. 

